Dinosaur soft tissues preserved as polymers

Soft tissue structures like blood vessels, tubular nerve projections, organic bone matrix, and osteocytes have been recovered from a variety of vertebrates, ranging from a Jurassic diplodocid (second row from top) to a modern Rhea (bottom row). Credit: Yale University/Nature Communications.

Since 2005, several samples of ostensibly soft tissue, such as blood vessels and bits of organic bone material, have been gleaned from dinosaur bones. The finds have stirred debate because the notion that intact dinosaur proteins could survive tens of millions of years has proved a tantalizing but difficult pill to swallow for many paleontologists. In a new study, however, researchers have identified a chemical pathway — well known in food science but not seen before in paleontology — that may be the key to long-term preservation of soft-tissue structures.

“According to the laws of chemistry and physics, the preservation of dinosaur proteins is completely paradoxical,” says Jasmina Wiemann, a doctoral candidate studying molecular paleobiology at Yale and lead author of the new study in Nature Communications. “Within a few hundred thousand to a million years, all proteins in soft tissue structures should be hydrolyzed and completely degraded.” And yet, teams have reported finding proteinaceous structures resembling bifurcating blood vessels, fibrous bone matrix and red blood cells in Mesozoic specimens including a 68-million-year-old Tyrannosaurus rex and an 80-million-year-old hadrosaur.

Wiemann and her colleagues characterized chemicals present in bones, teeth and eggshells from vertebrates dating from the Late Triassic to modern day. As expected, the modern samples all contained soft tissues. But a surprising number of fossil specimens also contained soft tissues, including structures resembling blood vessels, tubular nerve projections, collagen and bone matrix cells called osteocytes.

“We found preservational potential in all sorts of fossils,” Wiemann says. Using Raman microspectroscopy to identify the organic and inorganic contents of the soft tissue structures, the team found they were not made up of original proteins but instead had been chemically transformed into polymer compounds known as advanced glycoxidation end products (AGEs) and advanced lipoxidation end products (ALEs). Nonetheless, they were still recognizable as the original soft tissues.

“It was quite fascinating to see that different soft tissue structures were transformed more or less into the same type of compound,” Wiemann says. This transformation involves oxidative cross-linking of chemically reactive proteins with glucose or lipid molecules to form the polymers. AGEs and ALEs are well recognized in food science. “If you burn toast, the brown color that arises on the crust is due to the presence of these same compounds,” she says. The compounds help explain why many fossils are brown in color and why the chemical transformation helps preserve delicate soft tissue structures: The polymers are highly resistant to decay, water and bacteria.

“This study is a significant step forward,” says Matthew Collins, a biochemist at the University of York in England who was not involved in the new study. “These compounds are functioning as robust molecular replacements that preserve the original structures but not the protein molecules themselves.” This distinction helps explain why some biochemical tests, such as isotope and antibody tests, indicate that soft tissue samples from dinosaur fossils contain original material, while experiments involving protein sequencing have been unsuccessful.

Even if protein sequencing isn’t possible with the replaced tissues, Wiemann says, there are other tantalizing questions to ask of the tissues. “I have high hopes that we’ll be able to extract physiological and phylogenetic information [about dinosaurs] from these chemical transformation products.” For example, preserved blood vessels found in dinosaur eggshells might reveal new information about egg fertilization and embryo development.

The study offers some vindication for Mary Schweitzer, a molecular paleontologist at North Carolina State University, who has long contended that dinosaur soft-tissue samples that she and others have described are in fact endogenous material and not bacterial contaminants. “When you want to change the mindset of an entire discipline, it takes time. If this new study is the turning point, then I’ll be really happy,” Schweitzer says. “There’s just too much to learn to still be stuck on this same old question of whether soft tissues can preserve. There are a million things that these fossils are just sitting there waiting to tell us.”

The new study may also help scientists locate where more soft tissue-containing fossils may be uncovered: All the fossils found to contain soft-tissue structures came from oxidative environments with plentiful oxygen and slightly alkaline conditions. “If paleontologists want to find bones, teeth and eggshells with high potential for soft-tissue preservation, they should look for brown fossils in light-colored shallow marine limestones, river-deposited sandstones or fossil dune sands,” Wiemann says. Reducing environments that lack oxygen, such as deep marine deposits, are unlikely to yield soft tissue structures.

Wiemann and her colleagues tested this hypothesis at the Peabody Museum of Natural History at Yale. “We sorted through the collection by the type of depositional environment and in nearly every fossil recovered from oxidative settings we found soft tissues,” she says. “Now that we know where to look, I think we’re going to find a lot more examples of this kind of polymerized soft-tissue preservation.”